#define _CRT_SECURE_NO_WARNINGS 1

//https://leetcode.cn/problems/validate-binary-search-tree/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    long prev = LONG_MIN;
public:
    bool isValidBST(TreeNode* root) {
        return dfs(root);
    }

    bool dfs(TreeNode* root)
    {
        if (!root) return true;
        bool left = dfs(root->left);
        bool ret;
        if (prev < root->val)
        {
            prev = root->val;
            ret = true;
        }
        else ret = false;
        bool right = dfs(root->right);

        return left && ret && right;
    }
};